In tapping furnaces, ladles and other containers for molten metal, it is often difficult to decide when tapping is to be broken off. The metal is often covered by another phase, generally slag, which it is desired to avoid tapping with the metal in the last stage of the tapping process. This other phase--the slag--may be damaging to the continued process, or it may cause poor quality in the ready-cast metal.
It is generally extremely difficult to decide with the aid of the eye when slag instead of metal is starting to flow out, partly due to the high temperatures at which metals are processed, making both slag and metal red- to white-hot, partly due to the fact that both slag and steel are not transparent, and one can thus only see surfaces, and not what there is inside the flowing tap stream, for example, and partly due to the fact that the slag in view of vortex effects, i.e. whirl formation in the tap hole area, generally positions itself in the middle of the outgoing tap stream.
The above-mentioned difficulties of seeing when tapping is to be broken off, i.e. when slag instead of molten metal begins to flow out from the tap hole, or if tapping is done by tipping the furnace, ladle or the like, when slag begins to run with the metal, have caused methods to be sought after in the metallurgic industry for measuring when tapping is to be broken off, so that the maximum amount of molten metal can be obtained from the container while simultaneously tapping off slag is avoided to as great an extent as possible.
It is thus of great interest to accurately measure both remaining molten metal in the furnace and the amount of molten metal and/or slag flowing out during the final stage of tapping, i.e. that part of tapping when there is risk of slag accompanying the melt. Under practical conditions, the thickness or the depth of the remaining melt varies between 20 and 0.5 cm, depending on the size of the system, the diameter of the tap hole and the nature of the slag, in this interesting part of the tapping procedure which in continuation is denoted the final phase of the tapping procedure. The largest values are applicable to large containers and large tap holes (typical container diameter is 4 m, typical tap hole diameter 50 mm) and the smallest values for small containers (typical container diameter 2 dm, typical tap hole diameter 5 mm).
It has been attempted to weigh the process vessel and with the aid of a knowledge of the original weight of metal, it has been attempted to decide when the metal in the furnace, ladle etc. is coming to an end. Such a method is extremely uncertain, however, since there is no possibility of separating the weights of metal and slag, and since there is no possibility of deciding whether the furnace has been worn during the tapping process.
It has been further attempted to measure the vibrations of the tap nozzle, thereby to decide if molten metal or slag is running through it. However, the method is uncertain and has poor precision, for which reason no operational method has been able to be developed therefrom.
It is conceivable to measure the level of the molten metal, while utilizing the methods and apparatuses described in our patents U.S. Pat. No. 4,144,756 and U.S. Pat. No. 4,138,888 and GB No. 1,585,496. In doing so, the tapping procedure should be broken off on reaching a certain level of the molten metal. The bottom of the furnace, ladle or the like, from which tapping takes place, is generally subjected to wear, however, and such a method therefore would lead to different amounts of metal being left in the furnace, ladle etc., depending on the wear in the furnace etc.